PROJECT SUMMARY/ABSTRACT In this P01 proposal entitled ?Pathophysiology of genetically defined dementia and neurodegeneration: Defining therapeutic targets and pathways,? we seek to push forward the development of precise medicines to treat debilitating diseases associated with C9ORF72 G4C2 repeat expansions, the most common genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Success in developing a treatment for c9FTD/ALS will require a well-orchestrated effort that addresses multiple aspects of the drug discovery process. To improve the prognosis for patients suffering from c9FTD/ALS, we thus propose to investigate pathomechanisms by which C9ORF72 G4C2 repeat expansions cause disease, as well as develop bioactive small molecules and biomarkers. We have assembled a world- class team combining expertise in chemistry, neurology, cell biology, disease modeling, and biomarker development that has worked closely together and has all resources in place. Our significant progress to elucidate how expanded G4C2 repeat RNA transcripts drive toxicity and how to abrogate aberrant features associated with c9FTD/ALS has led to the discovery of: (i) novel pathomechanisms caused by the accumulation of G4C2 repeat RNA or ?c9RAN proteins? unconventionally translated from G4C2 repeat RNA; (ii) the first small molecule known to influence c9FTD/ALS disease biology; and (iii) a first-in- class biomarker to investigate new therapeutic strategies. We now bring forward novel and innovative chemical approaches to develop and optimize chemical probes to study and mitigate c9FTD/ALS disease mechanisms. We also present evidence that nucleocytoplasmic transport defects may be a fundamental pathway of c9FTD/ALS pathogenesis amenable to therapy. Indeed, we reported that disruption of the nuclear pore complex and nucleocytoplasmic transport is a primary cause of neurodegeneration in Drosophila and patient-derived cell models of c9FTD/ALS. In addition, we have established that poly(GP) c9RAN proteins are not only detectable in cerebrospinal fluid (CSF) from c9FTD/ALS patients but also in peripheral blood lymphocytes. What is more, preliminary data suggest that CSF poly(GP) levels associate with clinical features of disease. As such, poly(GP) proteins may prove useful in monitoring disease severity and rate of progression. Building upon these exciting findings, our multi-disciplinary studies will improve understanding of C9ORF72-related neurodegeneration, identify therapeutic targets and potential clinical and pharmacodynamic biomarkers, and lead to the design of bioactive small molecules with therapeutic potential. Combined, our efforts are anticipated to accelerate the discovery of an effective therapy for c9FTD/ALS.